Let's try and get this thread back on track, shall we, and ignore the troll.
So, why 'growth'?
The orthodoxy in economic theory puts growth front and centre because it is pretty evident that economic growth does give us clear benefits. This occurs almost by definition. Underpinning that growth is consumer demand. If there wasn't that demand, there wouldn't be growth. Which means someone somewhere wanted the additional production. Some previously unsatisfied want has been satisfied - therefore growth is a good thing.
In addition, growth allows us to take more leisure. The number of hours we work has fallen dramatically, believe it or not. Washing machines, Microwave ovens, etc. save us an enormous amount of time.
There are also some slightly more complicated reasons which basically show that growth > low unemployment, which is fundamentally a GOOD thing. People hate being unemployed, and it causes all sorts of political unrest.
There are numerous reasons why the emphasis on GDP is flawed though, and any economist worth his salt recognises this. (In fact, GDP only began in the 30s - every economist prior to that thought in terms of utility/happiness in a kind of Benthamite way)
One of the principal reasons why GDP's flawed is that there are loads of goods and services that don't get included in the numbers. For example, imagine a married couple with some kids. Currently the wife stays at home and looks after the kids, but then one day she decides she's bored and wants her career back. So they hire and nanny and she goes back to work. The additional production from her going back to work is genuine, but what about the nanny. The nanny's job was being done before by the wife, but there was no financial transaction in place. Has the nanny added anything? If the wife's labour of looking after the kids had been included in the GDP numbers, then hiring the nanny would not affect GDP, because her wage would be exactly equal to the theoretical wage of the housewife.
Similarly, i could wash my car for free, or i could pay someone to do it. If i'm lazy and go to the car wash, GDP will go up, but really its artificial.
Then there's all the environmental goods that don't get included - like say, clean air, or the reservoir of oil.
There's another equally fundamental problem, that some economists are waking up to (and, intriguingly, the Tories are flirting with them...), is that whilst GDP has grown massively over the latter half of the 20th Century, pretty much all the data available on the subject declares that we are no happier as a result.
The reason is not because having more stuff and money is bad - it's that we've done it in such a way as to make a lot of other things that affect our happiness change for the worse. Things like unemployment, the role of families, dislocation from local environment, increased crime, you know, the really fucking obvious stuff.
It's amazing that it's taken economists to wake up and smell the rotting sewers, but they are getting round to it. God knows if anyone in politics will pay the blindest bit of attention though. (As i say, Cameron has been flirting...eerghh...i'm skeptical.)
As a good (and very, very easy to read) intro the subject, you can check out:
Amazon product ASIN 0141016906
Ok, question 2: Why is population decline a problem?
Well, there are questions of international economic clout (fewer people = smaller economy) - eg America is fucking massive and has all the power, which is being threatened by China because it's growing so quickly. Remember, America's GDP PER HEAD is lower than many other countries, but it's total GDP is WAAY higher, which gives it its strength.
Also, a declining population also equals an aging population. Which means a BIG pensions liability. Which means people who are of working age get really fucked over paying for all the OAPs (since higher taxes are required).
That help?